Lead frames are used in the production of electronic devices mounted on printed circuit boards (surface mounted devices, SMDs). One step in the production of SMDs is the applications of a resin (mold) material on top of the lead frame for protection purposes, i.e. the formation of so-called packages. Lead frames usually contain copper and silver surfaces. Thus, the mold is in contact with silver and copper surfaces of the lead frame. During SMD product life time it must be guaranteed that there will occur no delamination between metal and mold, otherwise the SMD part may fail.
During the lifetime of the package, ambient moisture may be absorbed at the interface of mold and lead frame. The problem with moisture absorption and retention inside the package is that the trapped moisture will vaporize and exert tremendous internal package stress when the device is subjected to sudden elevated temperature, such as the solder temperature during board mounting, and this may lead to delamination. This moisture-induced delamination is called “popcorn effect”. To avoid the popcorn effect the packages must be packed or re-packed under moisture free conditions to avoid absorption prior to soldering, which makes the assembly process more costly and quality control more difficult. Due to the higher soldering temperatures used in lead-free soldering applications the risk of the popcorn effect occurring is particularly high, resulting in more package failure.
In recognition of the varying degrees of popcorn cracking tendency of various package types, IPC/JEDEC defined a standard classification of moisture sensitivity levels (MSLs) of leaded IC packages. According to this standard (J-STD-20 MSL), MSLs are expressed in terms of numbers, with the MSL number increasing with the vulnerability of the package to popcorn cracking. Thus, MSL1 correspond to packages that are immune to popcorn cracking regardless of exposure to moisture, while MSL5 and MSL6 devices are most prone to moisture-induced fracture. The target is to achieve MSL1.
According to the J-STD-20 MSL standard, the whole device is tested for a certain time under specified moisture and temperature conditions (see Table 1).
TABLE 1IPC/JEDEC J-STD-20 MSL ClassificationsSoak RequirementsFloor LifeStandard(1)AcceleratedConditionsTimeConditionsTimeConditionsLevelTime[° C./% RH][h][° C./% RH][h][° C./% RH]1unlimited≦30/85168 + 5/−085/85n/an/a21year≦30/60168 + 5/−085/60n/an/a2a4weeks≦30/60696 + 5/−030/60120 + 1/−0 60/603168hours≦30/60192 + 5/−030/60 40 + 1/−0 60/60472hours≦30/60 96 + 2/−030/60  20 + 0.5/−060/60548hours≦30/60 72 + 2/−030/60  15 + 0.5/−060/605a24hours≦30/60 48 + 2/−030/60  10 + 0.5/−060/606TOL(2)≦30/60TOL(2)30/60n/a60/60(1)The standard soak time is the sum of the default value of 24 h for the semiconductor manufacturer's exposure time (MET) between bake and bag and the floor life or maximum time allowed out of the bag at the end user or distributor's facility. For example, an MSL 3 package will require a standard soak time of 192 hours, which is 168 hours of floor life plus 24 hours between bake and bag at the semiconductor manufacturer.(2)TOL means “Time on Label”, i.e. the time indicated on the label of the packing.
In order to ensure sufficient adhesion under real life conditions, leaded IC packages are tested according to the IPC/JEDEC J-STD-20 MSL standard. Another practical test for adhesion strength is the so called Tab Pull Test which is common in the industry for qualification purposes. An indication of the strength of adhesion between a metal surface and mold may also be obtained from a simple Peeling Test. Both, the Tab Pull Test and the Peeling Test are used during the development and qualification phase as a good tool to identify improvements in adhesion between a metal surface and a resin material. The Tab Pull and the Peeling Test are typically performed on a test specimen rather than a real package. For the MSL test on real packages, C-Mode Scanning Acoustic Microscopy (C-SAM) can be used to detect delamination at the interface between silver and mold.
The achievable moisture sensitivity level depends not only on the adhesion between mold and lead frame surface but also on the package size and dimension. In general, SMDs are prone to popcorn cracking, because they are thin and therefore have lower fracture strength; they absorb and retain moisture more easily; and SMD board mounting also subjects the molding compound to the high temperature experienced by the leads.
The surface of most lead frames currently produced typically consist of two metals, namely copper or copper alloy, which is the lead frame base material, and silver. The relative proportions of copper and silver will vary between different lead frames. The base material influences the thermal and mechanical stability of the lead frame. Silver at the lead frame surface is required to create an electrically conducting connection between the lead frame and the chips mounted thereon. During further processing of lead frames, this is usually done by Thermo Sonic Bonding (TSB), which involves the contacting of a thin wire to both, the chip and the silver on the lead frame.
Thermo Sonic Bonding, which will also be referred to as wire bonding, is a surface-welding process in which two clean metal surfaces (substrate and wire) are brought into contact so as to create a stable bond between the bonding wire (which usually consists of gold, but may also consist of aluminium) and the silver on the lead frame substrate. Thus, this process is sensitive to impurities on the metal surfaces.
As far as copper and copper alloy surfaces of lead frames are concerned, it is now common in lead frame production to roughen the copper or copper alloy surface in order to improve the adhesion between that surface and the resin material (mold) subsequently used in the production of SMDs. The roughening is usually achieved by a chemical etching process; but it may also be achieved electrochemically, i.e., by applying an anodic current to the copper material. Some of the chemical etching processes also produce an oxide layer on the copper surfaces, which has a positive effect on adhesion, because metal oxide surfaces generally show better adhesion to resins than oxide-free metal surfaces. However, the effect of such an oxide layer is probably rather small compared to the effect of chemical roughening.
A standard process for roughening copper or copper alloy on the surface of lead frames and thereby improving adhesion of resin materials (molds) to the copper or copper alloy is the MoldPrep™ process developed by Atotech (see EP 1 820 884 A1). The MoldPrep™ process can be applied to lead frames which are already plated with silver. However, this process does not affect the silver surface. Thus, it has no influence on the adhesion of resin material to silver.
It has been found that, irrespective of how the copper material has been roughened, only the adhesion between copper and resin material is improved. None of the known processes for roughening copper or copper alloy surfaces results in a significant improvement of adhesion between silver and resin material. Therefore, the contact between silver and resin material is the weakest link between lead frames and resin materials and, thus, prevents further improvements in the MSL properties of SMDs.
In view of this situation, manufacturers of lead frames and SMDs are trying to minimize the silver surface area and maximize the relative proportion of copper surfaces in order to increase system stability. However, there are limits to this approach since certain minimum silver surface areas are required for contacting the chips to the lead frame surface.